Mixed chimerism induces donor-specific tolerance. However, graft-versus-host disease (GVHD) and failure of engraftment have limited the clinical application of this approach. Depletion of marrow T-cells, or transplantation of highly purified hematopoietic stem cells (HSC) into allogeneic recipients avoids GVHD, but the risk of graft failure is substantially increased. Non-HSC bone marrow populations increase engraftment. Bone marrow T cells eliminate alloreactive host cells and thus increase HSC engraftment in partially conditioned recipients. However, T-cell alloreactivity may also result in the death of the host through graft-versus-host disease (GVHD). We have discovered, and two independent labs have confirmed, a novel CD8+TCR- cell in bone marrow that facilitates engraftment of highly purified HSC in allogeneic recipients. A novel receptor consisting of the TCRbeta chain, CD3epsilon and an FC-specific "p33-FC" protein was recently discovered, and hypothesized to mediate direct HSC recognition by FC. Our preliminary gene-array analyses of CD8+TCR- FC revealed transcripts in common between FC and T cells, but only FC had message for TNFalpha. Moreover, our recent data indicate that FC from TNFalpha-/- mice are not functional. Our central hypothesis is that CD8+TCR- FC. unlike T cells, maintain long-term renewal of HSC through a TNFalpha-dependent mechanism, and that FC are derived from a common lymphoid progenitor (CLP) and as such share functional molecules with conventional T or NK cells, but are distinct from them. To test our hypothesis we will evaluate the role of FC and bone-marrow gamma and alphabeta+T-cells, and CD8alpha and CD8beta molecules in the maintenance of HSC long-term renewal in allogeneic engraftment. Moreover, we will determine the role of TNFalpha in FC-mediated engraftment and HSC survival through the use of mice deleted for TNFalpha and its receptors as a source of FC, HSC or as recipients in allogeneic transplantation. We will directly test the role of the FC in survival of HSC by 1) providing HSC with a transgenic-BCL2 survival molecule to determine if they become independent of FC in allogeneic transplantation, and 2) examining the cobblestone-area-forming capacity (CAFC) of HSC in marrow from mice we determine to be defective in FC function. Finally, we will use mice mutant for CD3epsilon, CD3delta, and CD3zeta in combination with lineage tracking of transplanted GFP-transgenic tagged HSC and lymphoid-progenitor cells to investigate the ontogeny of the FC and the biologic role of the CD3 complex in facilitation.